A great deal of effort has been devoted to devising what has been termed secondary recovery methods to increase the efficiency of extraction of oil from underground deposits. This problem is particularly apparent when recovering heavy crude oils within the range of 0.degree. to 20.degree. API. These oils are highly viscous and not generally productive in their natural state. Thus, an ongoing problem in the oil industry is the need to develop methods to lower the viscosity of oils prior to their being pumped from the underground deposit.
In the prior art, several processes for decreasing the viscosity of the crude oil in situ have been developed. For example, in the so-called "huff and puff" process, a high pressure steam is cyclically injected into a well followed by production phase recovery of the resultant lower viscosity oil. Viscosity can also be lowered by such methods as steam flooding, surfactant flooding, polymer flooding, in situ combustion and carbon dioxide flooding. These prior art processes are expensive and the cost of such flooding is usually excessive in relation to the oil production obtainable thereby.
Another way to lower the crude oil viscosity is to use a low viscosity diluent solvent which is extracted from the crude oil followed by reinjection back into the production well. The advantage of this approach is the continuous supply of fresh diluent solvents. U.S. Pat. No. 4,418,752 (Boyer, et al) is directed to a process for the production of heavy oil by injecting a diluent solvent down the production well to produce a blend having decreased viscosity. At the surface, the blend is treated in order to recover the solvent which is then recycled back into the production well to be mixed with the heavy oil. The diluent solvent is preferably a gas oil cut produced by fractional distillation of the recovered crude oil.
U.S. Pat. No. 4,284,139 (Sweany) discloses a process for upgrading oil production from a heavy oil reservoir wherein the heavy oil produced is combined with a diluent and subjected to thermal cracking. The thermally cracked products are fractionated to produce, inter alia the intermediate liquid and gas oil fractions, and a portion of the gas oil fractions is hydrogenerated with a hydrogen-containing gas stream to produce the diluent to be combined with the heavy crude oil. U.S. Pat. No. 4,362,212 (Schultz) discloses an enhanced oil recovery method wherein a liquid mixture of lower molecular weight hydrocarbons is injected into the underground deposit. The hydrocarbons are recovered from the mixture with the petroleum oil, separated by distillation and recycled back into the injection step. The diluent hydrocarbons employed in the process are those containing 2-5 carbon atoms.
In U.S. Pat. No. 4,883,582 (McCants) viscosity reduction is effected by using reactors for partially cracking the crude oil followed by mixing the products of the cracked crude oil with the untreated crude oil to yield a flowable, relatively low viscosity mixture.